A standard storage and/or transport container for fluent material is known comprising an erect and annular side wall and a bottom wall joined together at the outer edge of the bottom wall and lower edge of the side wall to form an upwardly open vessel, and a flexible bag or bladder within this vessel that lies against its inner surface and that itself contains the material being transported or stored. The side and bottom walls are typically made of round-section metal bars or rods that are spot-welded together in a criss-crossed gridwork with the bars welded at the intersections. It is also possible to use profiled bars and is in fact standard to provide a profiled rim element around the upper edge of the side wall. Frequently extra bars are integrated into the bottom or side wall for increased localized stiffness. In addition the floor of the container is often formed as a pallet that can be handled by a fork lift, and in fact in this case the floor can be made of wood while the sides are made of criss-crossed bars as described immediately above.
In order to empty such a container of a highly fluent material, for instance a liquid, it is standard to provide the liner with a drain fitting that projects from the rigid outer wall adjacent the floor and that itself incorporates a valve. The material inside the liner will empty out rapidly until the level is very low, that is down to the drain, and thereafter emptying will not only be quite slow, especially for a highly viscous liquid, but in fact some liquid will normally be left in the container. Even when the floor of the container is pitched somewhat toward the drain, the last phases of the emptying are invariably very slow due to the low hydrostatic pressure, and some liquid is often trapped in the container.